The present invention relates to apparatus for magnetically enhanced bias sputtering and plasma etching.
Sputter coating is a well-known technique for coating a substrate with material eroded from the cathode of a low pressure gas electrical discharge (glow discharge) created between a cathode and an anode maintained at a suitable voltage difference in a low pressure gas atmosphere. A glow discharge contains an abundance of positively charged ions formed by collision of electrons with atoms of the low pressure gas. The resulting ions are attracted to the negatively charged cathode, which they impact with considerable energy. This ion impact dislodges cathode surface atoms which will then condense on, and thereby coat, the surface of any object placed near the cathode.
Since sputtering is a low pressure process, it must be carried out in a hermetically sealed chamber, which is first evacuated and then back-filled with a suitable sputtering gas, usually argon, and maintained at the proper sputtering pressure, typically 5 to 40 millitorr.
In many coating applications a substrate to be coated is placed on the anode of the gas discharge, since the anode is usually directly opposite the cathode, in a suitable location for coating by dislodged cathode atoms. Most sputtering systems use an anode at ground potential and apply a large negative voltage to the cathode; the grounded sputtering chamber then becomes an auxiliary anode.
Bias sputtering is a modified sputter coating technique in which a bias potential, usually negative, is applied to the substrate which is to be coated. This bias potential causes some of the gas discharge ions to be attracted to the substrate during the deposition process. The ion impact can produce desirable changes in the nature of the sputter coating. An important use of the bias sputtering technique is in the so-called reactive sputtering process. During reactive sputtering, a chemically active gas, such as oxygen or nitrogen, is added to or substituted for the usual inert sputtering gas (e.g., argon). Reactive species of such active gas are created in the glow discharge region, along with the usual argon ions, and these species react with sputtered target atoms deposited on the substrate to form a desired compound. The reactive sputtering technique thus permits sputtering from a pure metal target, aluminum for example, to produce a compound coating on the substrate (e.g., aluminum oxide or aluminum nitride). Reactive sputtering has economic advantages because the sputtering rate from a metal target is much higher than from a target composed of the metallic compound.
Bias sputtering, by placing a negative potential on the substrate, increases the chemical reaction rate by, among other things, attracting the positively charged reactive gas species or ions. Substrate biasing has a limitation, however, because ion bombardment can also cause undesirable substrate heating and gas ion implantation in the coating. Thus, the problem is to obtain a large flux of low-energy ions (energy levels of 20 to 100 electron-volts) which are sufficient for the chemical reaction process at the substrate surface, without getting a significant amount of high-energy ion bombardment.
The same need to generate a large flux of low-energy ions is found in other plasma processes, such as plasma etching. Plasma etching is becoming increasingly important because it is superior to wet chemical processes for etching microscopic features, when used in conjunction with a suitable etch mask, in the manufacture of silicon integrated circuits. Present day very large scale integrated circuits (VLSI circuits), such as are used for semiconductor memories and processors, require a manufacturing capability to etch patterns having micron and even sub-micron dimensions.
The typical pattern etching procedure involves first applying a film of a photosensitive, X-ray sensitive, or electron-beam sensitive polymer (called a photoresist, X-ray resist, or electron-beam resist, according to the type of sensitivity) on the surface of a previously deposited layer which is to be etched. This polymer film is then selectively exposed to sensitizing radiation through a selectively opaque pattern or by modulated beam scanning.
Subsequent development of the exposed portions of the resist causes either the exposed or the unexposed portions to be removed, depending on whether the polymer is a positive resist or a negative resist. In either case, the resulting etch mask permits selective etching away of the portions of the underlying layer from which the resist was removed during development. This layer is usually a metal or a dielectric which serves some electrical function in the integrated circuit.
When etching is completed, the remaining resist material is removed by a resist stripping process, leaving behind the unetched portions of the underlying layer in the desired pattern. An integrated circuit is produced by repeated sequences of layer deposition, resist application, exposure, development, etching, and resist stripping.
Basic to each of these plasma processes is the creation of an electrical gas discharge (plasma) by imposing a direct current (dc) voltage or, preferably, a radio frequency (rf) voltage between electrodes in a space occupied by a normally non-reactive gas at low pressure. Energetic electrons emitted from the negative electrode (i.e., the cathode) collide with neutral gas atoms or molecules to create ions or other reactive species and additional electrons, thereby initiating and maintaining a highly conductive glow discharge in a region adjacent to the electrode. This glow discharge or plasma is separated from the electrode surface by a dark space or plasma sheath.
Since the plasma is essentially equipotential, the voltage drop between the plasma and the electrode occurs in the plasma sheath, and the direction of the electric field is normal to the electrode surface. Consequently, the ions and other reactive species generated in the plasma, which typically carry a positive charge, are attracted to the electrode surface and travel from the plasma to the surface primarily in a direction parallel to the electric field lines. In the plasma processes considered here, the electrode serves as a substrate support, so when the ions or reactive species reach the surface of the substrate they either activate or take part in chemical reactions resulting in the respective resist development, layer etching, and resist stripping.
The kinetic energy required for the chemical reactions involved in plasma processing are much lower, however, than the energies typically encountered in diode sputtering (several electron volts as compared with several hundred ev). The excess ion energy available in a sputtering system, therefore, would merely generate heat if used for plasma etching. This is highly undesirable because the polymeric materials used for etch masks cannot generally be used at temperatures above about 250.degree. C.
It is well known to increase plasma density in cathode sputtering processes by the use of a magnetic field. This causes a spiraling electron path and thus increases the probability of an ionizing collision with a gas molecule or atom. Particularly effective for increasing the ionization efficiency of plasmas are electron-trapping magnetic fields in which the lines of magnetic force cooperate with the electrode surfaces to form a completely enclosed region, preferably in which the magnetic field is orthogonal to the electric field.
It has been proposed to use magnetic enhancement also in lower energy plasma processes such as the bias sputtering and plasma etching processes described above. In one proposed arrangement, an electrode is formed with a prismatic body having several flat faces, constituting substrate support surfaces, arranged symmetrically about an axis. First and second magnetic pole pieces of opposite polarity project outwardly from the faces and extend completely around the electrode body at respective ends of the body, the resulting structure being basically spool-shaped. A magnetic field extending between the pole pieces thus forms a continuous belt around the body of the electrode adjacent to the substrate support surfaces.
The symmetrical prismatic spool shape of this previously proposed electrode provides multiple substrate support surfaces and is particularly suited to be mounted for rotation about its axis so that, in bias sputtering applications, each face can be directed in succession toward one or more sputtering targets. The prismatic shape also permits loading or processing a large number of substrates for a given size of electrode.
The symmetrical prismatic electrode must be centrally positioned in a vacuum chamber, however, and requires substrate holding devices because no more than one of the substrate support surfaces can be horizontal facing upwards. Many commercial sputtering systems, and particularly those used for integrated circuit production on ceramic wafers, are arranged to process the wafer substrates lying flat. A symmetrical prismatic electrode is not adapted for installation in such equipment.
In addition, the plasma region produced by such prismatic spool-shaped electrodes tends to be nonuniform, since the belt-like magnetic field bulges outward at its center region. This causes the plasma thickness to be greater at the center region than at the ends of the electrode body, thereby resulting in a nonuniform processing of the substrate surfaces.